As with any other dietary approach, results will vary and will be influenced by what it is you do eat.....we don't just eat for calories, but more importantly for essential nutrients - if we fail to meet our micronutrient needs, it doesn't much matter what we're eating, we're still malnourished at the end of the day.

That said, it is possible to eat a diet free of plant-based foods if you're paying attention to your nutrients (unless, of course, you're already part of a tribal culture that has figured out the finer details without knowing why it works) since it takes more than just the muscle meat and fat to meet nutrient requirements.....things like organs, eyeballs, bones, bone marrow, brain, etc. all contribute nutrients not found in the flesh and fat of the animal.....and eating a variety of animal foods is also helpful since eat land animal and/or fish/shellfish has different nutrient profiles, so you have a greater ability to meet nutrient needs with more than one animal in your dietary repetoire.

Even populations known to consume an animal diet consume some carbohydrate - even if it's a small amount.....so maybe a better distinction is plant-free diet or something along those lines since animal foods can and do provide some carbohydrate (depending on type of animal and part eaten).

Sustainability is a whole other issue - only the person attempting such a diet can say if it's sustainable or not....for some it can be, for others it's no way, no how.